Strip steel or sheet steel can be coated with lower melting metals, e.g. lead, by applying the molten metal to the steel substrate and permitting the molten metal to cool as a layer on the substrate and bond thereto.
Methods of cladding steel in this manner generally apply the molten metal to the steel strip or sheet substrate while the latter is inclined to the horizontal and induce solidification of the coating by forced cooling of the substrate and the applied materials.
In the system of German Pat. DE-PS No. 718,528, for example, the strip or sheet is passed through a channel which is inclined to the horizontal and into which the molten metal is poured.
In British Pat. No. 1,356,782, the material is applied from a funnel to the inclined substrate.
Both processes have been found to have a common disadvantage in that the feed of the substrate and the cooling of the applied molten material must be carefully coordinated and controlled with high precision if defects in the coating are to be excluded and malfunctions in the operation of the process are to be avoided.
Another disadvantage, particularly in the case of the system of German Pat. No. 718,528, is that the substrate to be coated, or the coating material, is limited as to the shape, extent or parameters of the cladding layer which can be fabricated.
It is known also to clad sheet or strip steel with lead (German patent document--Open Application DE-OS No. 20 08 454), introducing the substrate at an acute angle to the horizontal, from above, into a bath of molten lead overlain by a slag layer and then to draw the substrate, with the lead adhering thereto, through a die of appropriate shape determining the thickness of the cladding layer or layers.
Experience has shown that lead baths of this type suffer segregation and tend to develop inhomogeneities which may affect the coating.
Consequently, even after only a matter of hours, especially in the case of copper-alloyed lead, it is necessary to interrupt the operation, clean and empty the entire apparatus and then refill it before beginning again the cladding process.
Because of the time-consuming nature of the cleaning operation and the downtime of the apparatus during the cleaning procedure, this system has serious economic handicaps.
If cleaning is not carried out after sufficiently short intervals of cladding operation, the segregation brings about nonuniform coatings which result in warping and in irregular cladding.
Furthermore, experience has shown that the apparatus used in this system has disadvantages apart from those enumerated above in that, for example, it is difficult to satisfactorily seal the die against the leakage of lead and to restart the operation after the latter has been interrupted as is required after each cleaning sequence.
Finally, in connection with this arrangement it is noted that the system requires strong tensile forces to draw the substrate through the bath and the die, these forces frequently giving rise to differential changes in length between the lead layer and the substrate causing structural complications at the interfaces and in critical zones where bonding is required.